Nickel-plating bath for thorium



United States Patent C) 3,419,419 NICKEL-PLATING BATH FOR THORIUM RalphR. Wright, Oak Ridge, Tenn., assignor to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Filed Feb. 19, 1965, Ser. No. 445,865 1 Claim. (Cl. 117-130) ABSTRACT OFTHE DISCLOSURE A chemical reduction bath for nickel-plating thoriumhaving a pH of 9.0 to 9.5 and containing 0.05-0.15 mole per liternickelous sulfate, 0.3 to 0.6 mole per liter ammonium sulfate, 0.05 to0.30 mole per liter succinic acid, 0.1 to 0.4 mole per liter sodiumacetate, and 0.2 to 0.5 mole per liter sodium hypophosphite.

My invention relates in general to methods of coating a metallic articlewith a protective layer of another metal and specifically to methods ofcoating thorium with nickel.

A thorium body can be protected from corrosion by covering its surfacewith a metal which does not react with thorium or with the environmentto which the body is exposed. As is well known, various metals includingthorium can be nickel-plated by chemically reducing nickel in thepresence of the metal. In these chemical reduction methods a metal bodyto be plated is immersed in an aqueous solution containing nickel andhypophosphite ions under conditions whereby the nickel is reduced anddeposited in an adherent layer onto the surface of the metal.

The nickel-plating of thorium by this technique has :been only partiallysuccessful. The known nickel-plating =baths corrode the thorium surfacebefore an adherent protective layer of nickel is deposited thereon.Attempts have been made to plate thorium in accordance with the methoddisclosed in copending application Ser. No. 374,857, filed June 12,1964, now abandoned, for Nickel Plating Bath by Robert J. Clouse andJohn O. Dodson. While it has been possible to plate thorium using theplating bath disclosed therein, bodies so plated characteristically haveblack deposits at the thorium-nickel interface, and the nickel platingcan often be removed merely by scraping.

It is accordingly one object of my invention to provide an improvednickel-plating bath.

It is another object to provide a plating solution useful in depositingstrongly adherent nickel coatings onto thorium surfaces.

Other objects of my invention will be apparent from the followingdetailed description and the claims appended hereto.

In accordance with my invention I have provided a nickel-platingsolution having a pH of 9.0 to 9.5 and containing in the concentrationsindicated: nickelous sulfate, 0.05 to 0.15 mole per liter; ammoniumsulfate, 0.3 to 0.6 mole per liter; succinic acid, 0.05 to 0.3 mole perliter; sodium acetate, 0.1 to 0.4 mole per liter; and sodiumhypophosphite, 0.2 to 0.5 mole per liter.

This bath is free of ions such as fluoride and chloride which have atendency to corrode a thorium surface, and permits plating at arelatively low temperature, thus further minimizing thorium corrosion.An excellent bond between thorium and nickel is achieved by use of thisbath.

The process of plating nickel onto a thorium body is preceded by aprocess of cleaning the thorium body. This cleaning process must providean active thorium surface free of oxides, hydrides and other foreignmaterial. Steps of alternately electropolishing, rinsing and etchingwith acids will produce a surface having the desired properties.

The cleaned thorium body is then placed in the plating solution. Duringthe initial period of plating, i.e., until a ice continuous protectivefilm of nickel is on the thorium body, the plating solution ismaintained at a relatively low temperature, i.e., below about 30 C. andpreferably at a temperature of 22 to 28 C. At a temperature of about 25C. the nickel is deposited at a rate of about 0.055 mil per hour and at30 C. the rate is about 0.085 mil per hour.

While the entire coating of nickel may be deposited at these rates, itis normally desirable to deposit the final layer of nickel at a higherrate. This may be accomplished by plating an initial protective layer ofnickel onto the thorium in my bath and then either raising thetemperature of the bath or immersing the article in a conventional hightemperature electroless nickel-plating bath.

Having thus described my invention, the following examples are offeredto illustrate it in more detail. Example I gives the composition of mybath and Example II gives a method of using my bath including a methodof cleaning the thorium surface.

EXAMPLE I Grams per liter Moles per liter N ickelous sulfate 37 0. 14Ammonium sulfate 67 0.51 Suceinic acid 30 0.25 Sodium acetate (anhydrous30 0. 37 Sodium hypophosphite 50 0. 47

d A thorium coupon was cleaned by the following proceure:

1) The coupon was electropolished for 8 minutes in a bath containing 120ml. per liter 96% sulfuric acid and 500 ml. per liter phosphoric acid ata temperature of 55 C. and a current density of 2 amperes per squareinch using lead cathodes;

(2) The coupon was then raised in flowing water at less than 25 C. for 1minute;

(3) The coupon was then placed in a solution of 900 ml./liter glacialacetic acid and 100 ml./1iter 37 percent hydrochloric acid;

(4) The coupon was then etched for 7 minutes by placing it in a solutionof 3 normal nitric acid containing 23 grams per liter of a mixture ofpotassium dichromate and potassium fluoride which functioned as adeoxidizer;

(5) Step (2) was repeated;

(6) The coupon was anodically etched for 5 minutes in a 1.2 normal HClsolution at 25 C. and a current density of 0.4 ampere per square inchusing titanium cathodes;

(7) The coupon was then etched by placing it in a 3.6 normal H solutionat 25 C. for 5 minutes;

(8) Step (4) was carried out for 20 minutes;

(9) Step (2) was repeated;

(10) Step (6) was repeated;

(11) Step (7) was repeated;

( 12) Step (4) was carried out for 20 minutes;

(13) Step (2) was repeated;

(14) The coupon was pickled for 4 minutes in a solution of 58 rnl./literof 96% H 80 at 25 C.;

(15) Step (2) was repeated.

The cleaned coupon was placed in the bath of Example I at 22 to 28 C.for 2 hours while the bath was agitated. The resulting coupon having anickel coating about 0.13 mil thick was then placed in a conventionalnickel-plating bath maintained at 96 C., having a pH of 4.5, andcontaining in the concentrations indicated: nickel sulfate,

0.08 mole per liter; sodium hypophosphite, 0.23 mole per liter;propionic acid, 0.03 mole per liter; lactic acid, 0.03 mole per liter;tartaric acid, 500 p.p.m.; lead, less than 3 p.p.m.; and a wetting agentconsisting of a sulfonated saturated long-chain hydrocarbon, 50 ppm. Thecoupon was held in this hath until a plating 3.5 mils thick wasattained.

The resulting coupon was tested for adherence and continuity of thenickel coating by subjecting it to 100 percent relative humidity at 200F. At the end of 90 hours the coupon showed no evidence of failure. At900 hours the plating exhibited some discoloration, but showed no signsof peeling or porosity. At the end of 913 hours of exposure the platingburst from oxide build-up.

Other coupons plated as above were heated red hot with a torch and thenquenched in cold water. No bursting or peeling of the plating occurred.

The above examples are offered to illustrate, not to limit, myinvention. For example, the cleaning step does not constitute a part ofmy invention and is offered merely to show a procedure that may be usedto clean a thorium surface. My invention should be limited only inaccordance with the following claim.

Having thus described my invention I claim:

1. In a process for depositing nickel onto a thorium article the step ofimmersing said article into a nickel plating bath maintained at atemperature between 22 C. and 30 C., having a pH of 9.0 to 9.5 andconsisting essentially of the following in the concentrations indicated:nickelous sulfate, 0.05 to 0.15 mole per liter; ammonium sulfate, 0.3 to0.6 mole per liter; succinic acid, 0.05 to 0.30 mole per liter; sodiumacetate, 0.1 to 0.4 mole per liter; and sodium hypophosphite 0.2 to 0.5mole per liter.

References (Iited UNITED STATES PATENTS 2,532,283 12/1950 Brenner et a11061 2,819,187 1/1958 Gutzeit et al 117-430 2,916,401 12/1959 Puls eta1. 1061 JULIUS FROME, Primary Examiner.

L. HAYES, Assistant Examiner.

U.S. Cl. X.R. 1061

